1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an image forming apparatus, and more particularly, to a fixing device for fixing a toner image on a recording medium and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electro-static latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
Such fixing device may include a metal pipe to heat the fixing device effectively. FIG. 1 is a sectional view of a fixing device 20R including a metal pipe 202. The metal pipe 202 is provided inside a loop formed by a flexible fixing belt 201 to contact an inner circumferential surface of the fixing belt 201. A heater 204 provided inside the metal pipe 202 heats the metal pipe 202, and the heated metal pipe 202 heats the fixing belt 201.
The fixing belt 201 is pressed against a pressing roller 203 to form a planar or concave-shaped nip N between the fixing belt 201 and the pressing roller 203. As the fixing belt 201 rotates counterclockwise and the pressing roller 203 rotates clockwise in FIG. 1, the fixing belt 201 and the pressing roller 203 nip and convey a recording medium bearing a toner image while applying heat and pressure to the recording medium to fix the toner image on the recording medium.
Since the rotating fixing belt 201 is configured to slide over the stationary metal pipe 202, a lubricant is applied at a gap between the fixing belt 201 and the metal pipe 202 to reduce frictional resistance between the fixing belt 201 and the metal pipe 202. However, uneven expansion of the fixing belt 201 and the metal pipe 202 due to temperature deviation or uneven deformation of the fixing belt 201 due to pressure deviation at the nip N may shift the position of the fixing belt 201 relative to the metal pipe 202 in the axial direction of the fixing belt 201. As a result, the lubricant may leak from the gap between the fixing belt 201 and the metal pipe 202 at one or both axial ends of the fixing belt 201. Consequently, the lubricant may move onto the outer circumferential surface of the fixing belt 201 that contacts the pressing roller 203, resulting in slippage of the fixing belt 201 on the pressing roller 203. Moreover, the lubricant may farther move to the recording medium passing through the nip N, resulting in formation of a faulty toner image on the recording medium, or jamming or wrinkling of the recording medium.
On the other hand, after the flexible fixing belt 201 moves along the concave-shaped nip N that corresponds to the curvature of the pressing roller 203, the fixing belt 201 separates from the metal pipe 202 at a position downstream from the nip N in the direction of rotation of the fixing belt 201, creating an excessively large gap A between the fixing belt 201 and the metal pipe 202 as illustrated in FIG. 1 and causing the recording medium discharged from the nip N to contact the fixing belt 201 for longer time. Consequently, excessive heat is transmitted from the fixing belt 201 to the recording medium, generating hot offset of the toner image on the recording medium. Moreover, the large gap A may disturb separation of the recording medium from the fixing belt 201.